1. Field of the Invention
This invention relates to a semiconductor device, and in particular to a semiconductor device provided with a capacitor comprising a dielectric film made of a metal oxide, and to a method of manufacturing the semiconductor device.
2. Description of the Related Art
With a recent trend to increase the integration density of a semiconductor integrated circuit, the circuit pattern thereof becomes increasingly finer. For example, the size of cell for a capacitor has become extremely small. As the cell size of a capacitor becomes small in this manner, the capacitance of the capacitor will also be reduced correspondingly. However, there is a limitation on the reduction of the capacitance of the capacitor in view of sensitivity or soft error of the device.
In an attempt to solve this problem, there has been proposed to form a three-dimensional capacitor so as to increase the cell area and hence the capacitance as much as possible. However, there is a problem in this case that the working of a capacitor and the formation of electrodes or insulating film become increasingly difficult.
Under such circumstances, there has been studied recently to employ a film having a high dielectric constant as an insulating film for capacitor. Typical examples of such an insulating film having a high dielectric constant include a Ba.sub.x Sr.sub.1-x TiO.sub.3 film having a perovskite crystal structure. If this Ba.sub.x Sr.sub.1-x TiO.sub.3 film is to be employed, a noble metal such as platinum which is free from oxidation even in an oxidizing atmosphere is required to be used for a storage node electrode.
However, since the dry etching of such a noble metal is rather difficult, there has been studied to employ a metal oxide as the storage node electrode, which is capable of exhibiting almost the equivalent conductivity to a metal. Typical examples of such a metal oxide include ruthenium dioxide (RuO.sub.2). A method of forming a capacitor using this RuO.sub.2 will be explained as follows.
First of all, as shown in FIG. 1A, an isolation region 2 is formed in a p-type Si substrate 1, and then a gate insulating film 3a, a gate electrode (word line) 3b and an n.sup.+ diffusion region 4 of a transistor are formed. Thereafter, an insulating interlayer 5a is deposited to flatten the surface of the substrate and then a bit line 6 is formed. Subsequently, another insulating interlayer 5b is deposited again, a contact hole is formed, and then this contact hole is buried with an n.sup.+ type polycrystal Si film 7.
Then, as shown in FIG. 1B, a TiN film 14 is deposited as a diffusion barrier film (a barrier metal) and a RuO.sub.2 film 15 is deposited over the TiN film 14. Subsequently, a SiO.sub.2 film 16 is deposited as shown in FIG. 1C and then subjected to a photolithographic process to form a capacitor pattern.
Then, as shown in FIG. 1D, the RuO.sub.2 film 15 is processed by way of a reactive ion etching with the SiO.sub.2 film 16 being used as a mask, the SiO.sub.2 film 16 being subsequently removed by means of etching. Subsequently, as shown in FIG. 1E, the TiN film 14 is processed by way of a reactive ion etching with the RuO.sub.2 film 15 being used as a mask. Thereafter, as shown in FIG. 1F, the Ba.sub.x Sr.sub.1-x TiO.sub.3 film 9 is deposited as a capacitor insulating film and then heat-treated in an oxygen atmosphere. Finally, a WN.sub.x film 10 is deposited and worked to form an upper electrode.
However, the capacitor formed according to the aforementioned method is accompanied with the following problems. Namely, the surface of TiN film 14 constituting a diffusion-preventing film is oxidized in the step of forming the RuO.sub.2 film 15, forming a TiO.sub.x film 21 as shown in FIG. 2A thereby increasing the contact resistance. Alternatively, the RuO.sub.2 film is reduced and the TiN film is oxidized in the step of forming the Ba.sub.x Sr.sub.1-x TiO.sub.3 film 9 thereby increasing the contact resistance.
In an attempt to solve this problem, ruthenium (Ru) film 22 may be deposited in subsequent to the formation of the TiN film 14 constituting a diffusion-preventing film, and then the RuO.sub.2 film 15 is further deposited on the ruthenium (Ru) film 22. However, even in such a process, the side wall of the TiN film 14 is caused to be exposed after the working of electrodes so that the side wall of the TiN film 14 is oxidized as shown in FIG. 2B during the film-forming step of the Ba.sub.x Sr.sub.1-x TiO.sub.3 film 9 or during the subsequent heating treatment, thus increasing the leakage current or deteriorating the capacitance of the device.
On the other hand, when the RuO.sub.2 film 15 is directly contacted with the Si plug 7 as shown in FIG. 3A, a SiO.sub.2 film 23 is caused to be formed at an interface between the RuO.sub.2 film 15 and the Si plug 7 in the same manner as between the RuO.sub.2 film 15 and the TiN, thereby giving rise to an increase in contact resistance. Furthermore, if ruthenium is directly contacted with the Si plug, a ruthenium silicide 24 is caused to be formed as shown in FIG. 3B as a result of reaction between Ru and Si, and a SiO2 film 25 is caused to be formed between the Ba.sub.x Sr.sub.1-x TiO.sub.3 film and the ruthenium silicide 24, thereby giving rise to a problem of deteriorating the capacitance. Namely, it has been very difficult according to the conventional method to employ the RuO.sub.2 film or Ru film as a material for electrodes of a high dielectric capacitor.
As explained above, when a material having a high dielectric constant such as Ba.sub.x Sr.sub.1-x TiO.sub.3 is to be used as a material for a capacitor insulating film, a material which is capable of exhibiting a metallic conductivity even if it is oxidized, such as a ruthenium film or a ruthenium oxide film is required to be used as an electrode for a capacitor. However, the employment of such a material for an electrode is accompanied with the aforementioned problem that the electrode may be silicified as it is contacted with Si. On the other hand, if a diffusion-preventing film such as TiN is employed for the purpose of preventing the formation of a silicide, the diffusion-preventing film may be oxidized thereby giving rise to a problem of increasing the contact resistance.